This invention relates generally to actuators, and more particularly relates to techniques for actuating materials that demonstrate an actuation response to an applied stress.
Many advanced technical applications rely on actuation materials and actuation systems for implementing controlled motion and/or force generation in response to an actuation stimulus such as an applied stress. Popular classes of actuation materials include piezoelectric, magnetostrictive, and thermal and ferromagnetic shape memory alloys. Actuation systems based on these materials exhibit both performance advantages as well as limitations in actuation capabilities.
Piezoelectric materials are characterized by an ability to deform mechanically, i.e., expand and contract, in response to an applied electric field, as a result of the inverse piezoelectric effect. Piezoelectric ceramic actuators, commonly employed in series in the form of a stack, exhibit a very high actuation bandwidth, enabling a fast actuation stroke, while maintaining an acceptable output actuation energy density. Piezoelectric actuators are generally limited, however, to only a relatively small stroke extent, due in part to the brittleness of the piezoelectric ceramic, and in part to the limited strains produced at the maximum practical applied electric field. As a result, a stroke amplification mechanism is often required of a piezoelectric actuation system.
Magnetostrictive actuation materials in general can produce an actuation stroke extent and an actuation force that are greater than those of piezoelectric materials. Application of a magnetic field to a magnetostrictive material causes the material to be strained as the domain magnetization vectors of the material rotate to align with the direction of the applied magnetic field. The unit cells of the material are strained by the magnetization rotation but their orientation is not changed.
Magnetostrictive actuation elements are characterized by a fast actuation response time and by high actuation energy density. But magnetostrictive materials are fundamentally limited by their electrical conductivity, which in general precludes operation at very high actuation frequencies due to the formation of eddy currents in the material in response to a changing applied magnetic field. Like piezoelectric actuation materials, magnetostrictive actuation materials are also characterized by a limited actuation stroke extent, here due to limited domain elongation inherent in the actuation mechanism.
Classical shape memory alloys (SMAs) actuate as they proceed through a diffusionless transformation between a low-temperature, low-symmetry phase known as martensite and a high-temperature, high-symmetry phase known as austenite. In the martensitic phase, portions of the crystal, known as variants, having different crystal structure orientations, often form in pairs, referred to as twin variants. The boundary between twin variants is referred to as a twin boundary. Shifting of twin boundaries allows for low-stress deformation of the low-temperature martensitic phase, and is entirely reversible by returning to the high-temperature austenitic phase. This ability to thermally reverse large stress-induced martensitic deformation results in a large actuation stroke extent. The recoverable strain accommodated by a shape memory alloy is also quite large. Shape memory alloys can be made to act as cyclic or two-way actuators, in a process known as training. In one form of training, the material is cooled below the final martensitic transition temperature, Mf, and deformed to take the desired shape. The material is then heated to a temperature above the final austenitic transition temperature, Af, and subsequently allowed to take its austenite shape. The procedure is repeated multiple times, which completes the training. This process programs the material to take one shape when cooled, and another shape when heated.
Thermal control of the martensite-austenite SMA transformation severely limits the actuation response time of classical shape memory alloys, however. As a result, thermal shape memory actuation can not accommodate applications requiring even moderately high actuation frequencies. Thermal control of the shape memory effect also limits the operational temperature range of an actuation system.
Ferromagnetic shape memory alloys (FSMAs) are a subset of shape memory alloys that are characterized by a relatively large magnetocrystalline anisotropy and a low twinning stress in their martensitic phase. In the martensitic phase, twin variants having a magnetization vector that is less favorably oriented with respect to an applied magnetic field physically turn in relation to the field as their magnetization vectors are induced to align with the field. The resulting magnetically-controlled twin boundary motion requires no thermal transformation to the austenitic phase and produces a large actuation stroke extent. Ferromagnetic shape memory alloys are characterized by a moderately fast actuation response and correspondingly high-frequency operation at convenient operating temperatures, typically below 40° C.
The strength of the magnetic field required for ferromagnetic shape memory alloy actuation is in general not trivial to produce, however. Electromagnets designed to produce the required field continuously or with duty cycles greater than a few percent must be substantially larger than the actuation material itself; electromagnets built for continuous actuation must be hundreds of times the volume of a crystal to be actuated. The resulting bulk of a ferromagnetic shape memory alloy actuator prohibits its applicability for many actuation systems.
For many actuation applications, it is ideally preferred to achieve both the large actuation stroke of shape memory alloys and the fast actuation response time of magnetostrictive and piezoelectric materials. At the same time, the thermal constraint of classical shape memory, piezoelectric, and magnetostrictive materials, and the size requirement of ferromagnetic shape memory actuators are also preferably eliminated. Many advanced applications cannot be fulfilled until a single actuation system can address all of these considerations under practical operating conditions.